This invention relates generally to clay pigments useful in the production of coated printing paper. In particular, this invention is concerned with a novel kaolin clay pigment composition adapted for use in the production of coated printing papers having dull and matte finishes.
A wide variety of coated printing papers is produced commercially. Such papers are prepared by coating suitable basestock with a pigmented coating composition. The coating composition is frequently called a "coating color." Among the principal properties of paper which are affected by applying the coating are smoothness, ink receptivity and gloss.
In recent years most of the coated paper has been used for advertising matter, periodicals, etc. This type of paper usually has a high gloss, i.e., paper having a TAPPI (75.degree.) gloss of 50% or above. The glossy finish is desired to emphasize contrasts in half-tone prints and to satisfy aesthetic demands. Fine particle size kaolin clay, e.g., clay that is at least 80% by weight finer than 2 .mu.m, e.s.d., is usually the main or sole constituent of the pigment employed in the production of high-gloss coated printing paper. A coating color containing the fine particle size clay pigment is applied to suitable base paper. The coated paper is dried and subjected to super-calendering. Such calendering serves to produce a smooth surface which responds well to printing. Calendering also serves to develop the desired high-gloss finish.
There is an increasing demand for coated printing papers which have a lower gloss-finish than the present glossy sheets. These lower-gloss sheets are of two general types. One is a dull-finish sheet. Dull-finish is generally defined as a paper which has a TAPPI (75.degree.) gloss in the range of about 20% to 35%. Presently available dull-finish printing paper is obtained by using a blend of a calcium carbonate pigment, usually a precipitated grade, with a conventional coating grade of kaolin clay, e.g., kaolin clay that is about 80% by weight finer than 2 .mu.m. Coating is followed by a finishing operation which is usually a light calendering operation. This necessitates the use of highly specialized calendering techniques and/or equipment. The finishing operation is used to increase the smoothness of the surface of the sheet to facilitate printing. The degree of calendering is less than that which would produce optimum printing smoothness. Consequently, special quality paper basestocks are usually used in the production of dull-finish printing paper. The second type of low-gloss paper is so-called "matte" finish paper. Matte coated paper has a TAPPI gloss in the range of 0 to 15% and receives no calendering. Special basestocks are used to prepare matte-finish paper. It has been reported that the pigments used are similar to those employed in the production of conventional coated printing paper.
The prior art pigments capable of producing dull and matte-finish sheets are detrimental to one or more characteristics of the sheet. Generally, available dull and matte-finish coated paper lacks the printing properties of clay-coated glossy sheets. The most serious deficiencies of available dull and matte sheets, when compared to high gloss coated paper, are ink mottle, low surface smoothness and inadequate ink holdout (inadequate gloss of the printed image and excessive ink strike-in). Low surface smoothness and inadequate ink holdout are most severe in the case of matte-finish sheets which are not calendered and thus are not smoothed during manufacture.